The present invention relates to an optical disc and more particularly to an optically read information recording medium within which a space is formed by a pair of disc substrates and spacers that are interposed between the disc substrates.
In an optical disc such as a DRAW (direct read after write), information is recorded as a number of fine pits. It is therefore significant or important to protect a layer where the pits are formed, i.e., a signal recording layer. In order to meet this requirement, there have been developed and proposed optical discs in which a pair of disc substrates are arranged in confronted relation with each other with a signal recording layer of at least one of the pair of disc substrates being directed inwardly.
FIG. 1 shows a typical example of a conventional optical disc. As shown, a pair of disc substrates 1 and 2 made of transparent resinous material are provided, respectively, with signal recording layers 3 and 4. The disc substrates 1 and 2 are arranged with their signal recording layers 3 and 4 being confronted with each other and are bonded through inner and outer spacers 5 and 6 with, for example, adhesives. Thus, an aerial space 7 is formed between the two disc substrates 1 and 2, thereby protecting the signal recording layers 3 and 4. A vent hole 8 is formed in the outer spacer 6 so that the space 7 is in communication with the outside through the vent hole 8.
In such an optical disc, since atmospheric air may flow through the vent hole 8 into the space 7, even if the atmospheric pressure is changed, there is an equilibrium between the atmospheric pressure and a pressure within the space 7. Thus, undesired warpages in the disc substrates 1 and 2 may be avoided during the change in atmospheric pressure. However, in contrast, there is raised a problem in that foreign matter such as dust will enter into the space 7 when the atmospheric air is introduced into the space 7 through the vent hole 8. Although FIG. 1 shows a thickness of the optical disc on an enlarged scale, the inner and outer spacers 5 and 6 are actually very thin. It is, therefore, difficult to form the vent hole 8 in such a restricted or narrowed space.